1. Field of the Invention
This invention relates to a carriage driving method for driving a carriage using a motor.
2. Description of the Prior Art
In a conventional carriage driving device, the driving force of a motor is transmitted to a carriage via a belt or a wire to reciprocate the carriage at a constant speed. Such a driving device is used in, for example, a printer device mounting an ink-jet head on a carriage, an original reading device mounting a short-focus image pickup device and an optical sensor on a carriage, or the like. In general, the angular velocity of the motor is changed until the carriage is driven at a constant speed by driving the motor with constant angular acceleration or so that a predetermined constant angular velocity is obtained when the angular acceleration becomes zero while gradually reducing the angular acceleration.
However, in the above-described conventional device, periodic vibration of the carriage in its moving direction is generated when the carriage in a stopped state starts to move, and the carriage is accelerated to a constant speed while being accompanied by the vibration. Hence, the conventional device has the following disadvantages.
That is, although gradually attenuated, the vibration of the carriage is not completely attenuated even if the carriage driving motor is rotating at a constant angular velocity. Accordingly, a long time period is needed until the carriage moves at a constant speed, and therefore a long distance is also needed to attain the constant speed.
A waveform diagram shown in FIG. 8 illustrates the above-described disadvantages in the conventional device. FIG. 8 represents a case in which a carriage is driven by a stepping motor having a step of 0.9.degree. and a pulley having a pitch circle whose circumference is 50 mm (a diameter of 15.915 mm) via a belt, and is accelerated to a constant speed of 250 mm/sec (with a driving frequency of the stepping motor of 2000 pps, 50 mm.times.2000 pps.times.0.9.degree./360.degree.=250 mm/sec). In FIG. 8, the driving frequency of the stepping motor, and the speed of the carriage are represented by a broken line curve a" and a solid-line curve b", respectively, while the ordinate represents the driving speed of the stepping motor and the speed of the carriage, and the abscissa represents time.
As is apparent from FIG. 8, the drive of the stepping motor is suddenly started at least at a certain frequency (400 pps in the present case) depending on the characteristics of the motor. At that time, the carriage abruptly starts to move from the speed 0. Since the curve of the driving frequency of the motor is less steep than the curve of the speed of the carriage as indicated by the broken line, the speed of the carriage "b" exceeds the curve of the driving frequency of the motor at point b".sub.1, where the carriage starts to vibrate.
It can be easily understood that the vibration remains without being completely attenuated at a time period when the speed of the motor reaches the range of a constant speed.
In conventional techniques, in order to promptly attenuate the vibration of the carriage, it is necessary to increase the sliding force between the carriage and the bearing member, or, as disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 58-179675 (1983), to increase resistance by providing the carriage with a sliding force due to another sliding member. However, such sliding resistance has many unstable factors, such as variations in environment of use, or the like, causing variations in the load, which will be a factor causing unevenness in the speed of the carriage.
In order to overcome such problems, two types of carriage driving methods have been proposed in which periodic vibration of a carriage is promptly attenuated without increasing the sliding force, whereby a constant speed can be attained from a stopped state in a short time and in a short distance.
One of such carriage driving methods is disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 58-185284 (1983). In this method, driving pulses having a frequency lower than a rated frequency are supplied to a carriage driving motor from the start of the drive until the value of acceleration is inverted from a positive value to a negative value, and driving pulses with the rated frequency are supplied thereafter.
The other method is disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 2-145370 (1990). In this method, while acceleration of a carriage is reduced from zero to a negative value after the start of the drive and thereafter tends to increase, the driving frequency of the motor is increased at the moment when acceleration is zero.